1. Field of the Invention
The present invention relates to an Ultra-Wideband (UWB) wireless communication system in a multi-user environment, and more particularly, to a UWB wireless communication system that can determine a threshold value to eliminate a signal pulse affected by a near-interference from among a plurality of received signal pulses.
2. Discussion of the Background
An Ultra-Wideband (UWB) technology can transmit data via a comparatively wide frequency band, and has been considered as a promising technology for a wireless personal area network (WPAN), which is generally used to exchange various types of data between two or more digital devices that are located within a short distance from each other.
A UWB wireless communication system uses a wide frequency band, which includes a commercially used frequency band. Thus, a method for receiving a plurality of signal pulses and estimating a bit value at a receiver by receiving a signal pulse corresponding to each bit a predetermined number of times is commonly used to reduce the interference that may affect signals transmitted in a commercially used frequency band.
As described above, since signal pulses are quickly transmitted for a comparatively very short time, it is very important to synchronize an originating transmitter and a target receiver. However, even when synchronized, a delay may occur in a multi-path wireless communication system.
Specifically, in a multi-user environment where the UWB wireless communication system includes a plurality of transceivers, a signal transmitted from an interfering transmitter other than the originating transmitter and received at a target receiver may act as an interference signal for the target receiver due to the failure of synchronization and the delay spread. Here, the interfering transmitter may be located closer to the target receiver than an originating transmitter that is transmitting the signal intended to be received by the target receiver. Also, since the interfering transmitter is located closer to the target receiver, an interference signal may have a greater effect on a bit decision than a signal transmitted from the originating transmitter. Accordingly, the above-described problem becomes more serious.
More specifically, an interference signal from the interfering transmitter located closer to a target receiver may have a greater received power than the received power of a signal from an originating transmitter. This is because received power of a signal is a function of a distance between the transmitter and the receiver. Thus, data from the originating transmitter may not be received by the target receiver. This is referred to as a near/far problem.
The near/far problem may occur in different wireless communication systems. To solve the near/far problem in a Code Division Multiple Access (CDMA) system, uplink data is transmitted at a varying power depending on a distance between the mobile stations and a base station. Therefore, a signal transmitted from the base station has a uniform received power at the mobile stations irrespective of the distance between the mobile stations and the base station.
However, even when an originating transmitter controls transmission power, an interference signal at a receiver, including a target receiver, may not be controlled. Thus, the above-described method of solving the near/far problem may not be applicable to the UWB wireless communication system in the multi-user environment.
Rather, studies to solve the far/near problem for the UWB wireless communication system in the multi-user environment have been classified generally into two schemes.
A first scheme is referred to as “spatial exclusion”. Spatial exclusion divides the entire space into domains or exclusion regions surrounding the receivers. Near-interference from transmissions outside a domain may be ignored by a target receiver inside the domain. Then, scheduling is performed so that an originating transmitter does not transmit a signal when it is located within a domain of a receiver that is not the target receiver. Thus, the generation of near-interference may be reduced.
However, in spatial exclusion, a performance of the receiver is globally and locally affected by a domain radius from the receiver. For example, if the domain radius is too large, generation of near-interference may be significantly reduced, but transmissions may be hindered. Conversely, if the domain radius is too small, more simultaneous transmissions may occur at the expense of increased near-interference. Thus, the domain radius may not be simply defined. Also, when a transmitted/received power level is low, as in the UWB wireless communication system, the power of a signal may not be quickly measured. Accordingly, a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) scheme for detecting signal transmission by another transmitter located within a domain to avoid a collision may not be applicable. Also, due to a domain management and scheduling between many links in a domain, a great amount of control overhead is necessary. This overhead can be prohibitively high for a UWB wireless communication system that operates with limited available resources.
A second scheme is referred to as a “temporal exclusion”. Temporal exclusion divides a single frame into many time slots and performs scheduling so that a signal transmitted by an originating transmitter and a signal transmitted by an interfering transmitter are transmitted in different times slots.
However, temporal exclusion also requires overhead and a central manager to assign and manage allocation of time slots. In an ad-hoc network, a transceiver is required to function as a central manager. The transceiver functioning as the central manager incurs an extra computing burden to assign and manage time slots, and also consumes additional power. Thus, a battery-operated portable device may not be suitable as a central manager. Also, as described above, in the UWB wireless communication system, many signals are transmitted at a very high speed. Thus, when time synchronization has even a small error, inaccurate bit information may be transmitted. Also, bit information may become an interference signal in a different time slot. Specifically, temporal exclusion is required to be very precisely embodied. Thus, adaptability of temporal exclusion is decreased. Also, as with spatial exclusion, a great amount of control overhead is needed for temporal exclusion. This overhead can be prohibitively high for a UWB wireless communication system that operates with limited available resources.
As described above, conventional methods for solving the near/far problem in the UWB wireless communication system in the multi-user environment may not be readily embodied. Even when the conventional methods are embodied, satisfactory performance may not be possible.
Accordingly, a new technology that can reduce the above-described near/far problem and can be readily embodied is required.